In a large bowl, combine the cabbage, carrots, yogurt, vinegar, buttermilk and sugar and toss to combine. Season to taste with salt and pepper. Cover and refrigerate at least 30 minutes.
Meanwhile, pull the meat from the rotisserie chicken and shred with your fingers or two forks (discard the skin and bones). Toss the chicken with the barbecue sauce to coat, adding more if necessary. If you would like, warm the chicken in the microwave until heated through. Pile the chicken on hamburger buns and top with the coleslaw. Serve with a pickle if desired.